Synthetic polymeric filaments which are to be cut into staple then twisted into staple yarn usually must be crimped before cutting so that the staple will behave properly in the drafting and twisting operations. Crimp is also needed to contribute bulk, softness and insulating ability to yarns, either staple or continuous filament, which are to be used for garments, upholstery or carpets. The twisting operation tends to compact the filaments and straighten the crimp. It is desirable that filaments have crimp which resists such compacting and straightening. Such filaments are usually crimped by a mechanical stuffer box, in which nip rollers force the filaments into a chamber having a means to impede their exit so that the filaments are forced to bend in a zig-zag manner as they encounter a mass of previously crimped material. The filaments are heated by various means as they are crimped, then and as they cool when leaving the stuffer box a considerable portion of the crimp which they receive is retained.
Most useful polymeric filaments resist permanent deformation during crimping and tend to spring back toward their original straight condition, thus minimizing the sharpness of the bends and the resultant degree of bulk exhibited by the final yarns. When the bending modulus of filaments is lowered in an attempt to obtain sharper bends, such as by orienting the filaments less or by copolymerizing, it is usually found that such filaments also tend to lose their crimp more readily when they are tensioned in handling after crimping or when they are drafted, twisted and subjected to normal tension of use in fabric form.